In such capacitive sensors, the common moving electrode forms part for example of an armature elastically held between the two fixed electrodes. This common electrode is capable of moving some distance in the direction of one or other of the fixed electrodes via the action of a force or a pressure, for example. At rest, this common electrode is normally equidistant from the two fixed electrodes. This defines equal capacitive values for the two capacitors. When the common electrode moves via the action of a force or pressure or another physical quantity to be measured, the capacitive value of each sensor varies inversely. The interface circuit connected to the capacitive sensor is thus for providing an output signal in the form of a voltage depending upon the variation in the capacitances of the two capacitors.
The output voltage may vary linearly in an ideal case relative to the movement of the common moving electrode, which is not theoretically entirely the case if the interface circuit is integrated into a semiconductor substrate. Stray capacitors, which are added to the capacitors of the sensor, normally have to be taken into account. These stray capacitors are practically independent of the movement of the common electrode, which can create certain non-linearities. The same is true for a MEMS type sensor integrated in a semiconductor substrate, which is connected to the interface circuit to form the electronic circuit. Consequently, the electronic circuit output voltage cannot vary linearly in accordance with the movement of the common moving electrode as a function of an electrostatic force to be measured.
Generally, in order to measure a force, acceleration or pressure, the fixed electrodes of the two capacitors are polarized or excited cyclically by voltages of opposite polarity relative to an intermediate or median voltage at rest. These two polarizing voltages are for example that provided directly at the terminals of a continuous voltage source, such as a battery, namely a low voltage, which can be earth, and a high voltage. By polarizing the two fixed electrodes at different voltage levels, the charge difference across the moving electrode can be measured and converted into an output voltage of the electronic circuit. Several cycles or successive measuring periods are necessary and are each divided at least into a phase of polarizing the fixed electrodes and a phase of discharging the fixed electrodes by the output voltage. When the output voltage is stabilised at its final value, the total charge transfer of the moving electrode becomes zero. This output voltage can be provided, sampled, to a processing circuit able to provide acceleration, force, pressure or angular velocity data depending upon the structure of the sensor.
In order to check the proper operation of this type of electronic circuit with a capacitive sensor, a functionality test must be carried out either at the beginning of the operation thereof, or during force, acceleration, or pressure measuring cycles. In order to do this, an electrostatic force is deliberately and cyclically generated replacing, at least one in two times, a polarizing phase with a test phase. In this test phase, a single fixed electrode of one of the capacitors is polarized for example at the high voltage of the continuous voltage source without polarizing the other fixed electrode. Because of this unbalance, a mechanical force is thus produced on the moving electrode which will tend to be directed towards the polarized fixed electrode. At the end of this automatic test, the stabilised output voltage value enables the electrostatic force, deliberately generated for the electronic circuit operation check, to be estimated. If this test was carried out during measurement of a real force, the value of the output voltage will cumulate or subtract the dependence of the real force and the generated electrostatic force.
One embodiment of an electronic circuit with a capacitive sensor for implementing the automatic test method is shown in FIG. 1, in addition to the various measuring cycle phases illustrated by a time diagram of voltage signals at the sensor in FIG. 2. The electronic circuit shown is based on an electronic circuit disclosed in the article by Messrs H Leutold and F Rudolph, which appeared in the review entitled, “Sensors and actuators” A21-A23 (1990), pages 278 to 281, and also in FR Patent No. 2 720 510.
The electronic circuit 1 shown includes a capacitive sensor 2 and an interface circuit connected to the sensor for providing a measuring output signal Vm. Capacitive sensor 2 includes two differential mounted capacitors having a common electrode Cm able to move between two fixed electrodes to define two capacitors C1 and C2. The electronic interface of sensor 2 of electronic circuit 1 includes a charge transfer amplifier unit 4, which is connected at input to the common electrode Cm, an integrator unit 5 for permanently providing at output a voltage Vm equal to the integral of the charges provided by amplifier unit 4, and an excitation unit 3 for cyclically polarizing the fixed electrodes at determined voltage levels.
The charge transfer amplifier unit 4 includes an operational amplifier 10, three capacitors C3, C4 and C5 and two switches 16 and 17. The inverting input of this amplifier is connected to common electrode Cm. The capacitor C3 in parallel with switch 16 are connected between the inverting input and the output of amplifier 10. Capacitor C4 is connected between the output of amplifier 10 and the input of integrator unit 5. Capacitor C5 is connected between the non-inverting input and a voltage reference terminal Vref, which may be for example equal to VSS. Finally, switch 17 is arranged between the output of integrator unit 5 and the non-inverting input of amplifier 10.
Integrator unit 5, which follows charge transfer amplifier unit 4, includes two input switches 18 and 19, an operational amplifier 11 and an integration capacitor Cf. This capacitor Cf is connected between the inverting input and the output of amplifier 11, which supplies the output voltage Vm of integrator 5. The input switch 18 is arranged between the output terminal of capacitor C4 of charge transfer unit and the non-inverting input of amplifier 11. The potential of this non-inverting input of amplifier 11 is fixed at the voltage reference Vref. Switch 19 is arranged between the output terminal of capacitor C4 of charge transfer unit 4 and the inverting input of amplifier 11.
Excitation unit 3 includes four switches 12, 13, 14 and 15, which can be formed by MOS switching transistors in the integrated circuit. The first switch 12 is arranged between the output of integrator 5 and the fixed electrode of capacitor C1. The second switch 13 is arranged between the output of the integrator and the fixed electrode of capacitor C2. The third switch 14 is arranged between the high voltage terminal VDD of a continuous voltage source and the fixed electrode of capacitor C1. Finally, the fourth switch 15 is arranged between the low voltage terminal VSS of the voltage source and the fixed electrode of capacitor C2.
In the normal electronic circuit operating mode, each cycle or successive measuring period is divided into two successive phases P1 and P2 as shown in part in FIG. 2. Passage from one phase to the other is controlled by clock signals that are not shown so as respectively to open or close the switches. Each switch is switched on or off taking account of time overlap or non-overlap conditions for each phase. For the first phase P1, switches 12 and 13 are closed (switched on) in order to discharge the two capacitors completely using output voltage Vm as shown by the diagrams of voltages VC1, VCm and VC2. Switches 16, 17 and 18 are also closed, whereas switches 14 and 15 are open (switched off) in this first phase P1. In the second phase P2, switches 14 and 15 are closed, whereas switches 12, 13, 16, 17 and 18 are open. In this second phase P2, voltage VDD is applied to the fixed electrode C1 seen in the VC1 diagram, whereas voltage VSS is applied to the fixed electrode C2 seen in the VC2 diagram. If the moving electrode has moved a certain distance in the direction of one or other of the fixed electrodes, the capacitances of the capacitors will vary inversely. This will lead to a difference in the charge accumulated by each capacitor, which also depends upon the voltage Vm applied previously to each electrode of the capacitors.
In order to carry out the automatic test of electronic circuit 1 in a conventional manner, an electrostatic force has to be generated electronically to emulate, for example, an acceleration or another physical quantity. In order to do this in the measurement cycles, the second phase P2 is replaced at least once every two successive cycles by a test phase TF as shown in FIG. 2. In this test phase TF, only the fixed electrode of capacitor C1 is polarized at the high voltage VDD, whereas the fixed electrode of capacitor C2 is kept at the value of the preceding output voltage Vm, just like moving electrode Cm. In this test phase TF, only switch 14 is closed, since there is no next integration operation when this electrostatic force is generated. The moving electrode is deliberately mechanically drawn towards the single polarized fixed electrode of capacitor C1, which emulates, for example, an acceleration to be measured. The movement of this moving electrode, due to the force generated during each test phase TF, is thus measured in the measurement cycles via a second measuring phase P2 that occurs once every two times alternately with each test phase.
After several operating cycles for the automatic test, the final value of voltage Vm supplies an indication of the deliberately generated electrostatic force. FIG. 2 only shows the final stabilised Vm value shown in voltage diagram VCm. Since the moving electrode has moved in the direction of the fixed electrode of capacitor C1, this output voltage Vm is above the intermediate or median rest voltage (VDD+VSS)/2.
It will be noted that with an automatic operating test of the electronic circuit as described above, the electrostatic force generated is obtained in each test phase by polarizing only one of the fixed capacitor electrodes via the high voltage of a continuous voltage source. This polarization of one of the fixed electrodes could also be carried out with the low voltage. Consequently, this electrostatic force is greatly dependent upon the high or low voltage value which may vary given that it is supplied by a battery, which can be discharged, which is a drawback. Moreover, with an automatic test of the prior art, it is not possible to adjust or program a determined electrostatic force as a function of any type of capacitive sensor to be tested, which constitutes another drawback.